In recent radio communications systems, instead of accommodating a single modulation and encoding system, using an adaptive modulation and code rate communications system that controls so as to perform communications by way of an optimum system depending on the conditions for communications is proposed.
Communications systems using adaptive modulation and code rate (hereafter, referred to as an adaptive encoding modulation system where appropriate) change the code rate of an error correction code and the degree of multi-valued modulation depending on the quality of a propagation path, and provides high-speed data communications to a user, whose propagation path has a high quality, at the expense of noise durability characteristics, and provide low-speed data communications to a user, whose propagation path has poor quality, putting noise durability characteristics ahead.
Such adaptive encoding modulation systems are expected to be additionally employed even in W-CDMA (Wideband-Code Division Multiple Access) which is attracting attention as a third generation mobile communications system.
In adaptive encoding modulation systems, adaptive modulation and code rate between a base station and a terminal is realized through the basic procedures below.    1. The terminal measures the reception quality of a signal transmitted from the base station.    2. The terminal returns to the base station a reception quality message that indicates the measured result of the reception quality.    3. The base station determines the optimal modulation system and code rate from the reception quality message transmitted from the terminal, and transmits to the terminal a transmission parameter that indicates the determined modulation system and code rate.    4. The base station transmits user data based on the transmission parameter.    5. The terminal receives the transmission parameter and carries out a data receiving process based on the transmission parameter.    6. 1˜5 mentioned above are repeated periodically.
An outline illustrating this procedure is shown in FIG. 1. In FIG. 1, the relationship between a downlink control channel for notifying the transmission parameter from the base station to the terminal, a downlink data channel for transmitting the user data from the base station to the terminal and an uplink control channel for transmitting the reception quality message from the terminal is shown. In the present figure, an example in which the steps 1˜5 mentioned above are carried out at predetermined frame periods is shown.
That is, in FIG. 1, the terminal measures the present reception quality at the terminal and transmits a reception quality message indicating the reception quality to the base station through the uplink control channel.
The base station determines, from the reception quality message transmitted from the terminal, a combination of modulation method and code rate with which, for example, the error rate of the received data at the terminal is at or below a predetermined value, and transmits, as a transmission parameter, information that indicates the modulation method and the code rate to the terminal through the downlink control channel. Moreover, the base station transmits user data to the terminal through the downlink data channel in accordance with the code rate and the modulation method corresponding to the transmission parameter transmitted to the terminal.
Then, the terminal receives the transmission parameter transmitted from the base station in advance and thereby recognizes the code rate, the modulation method and the like of the data transmitted from the base station thereafter. Moreover, the terminal receives the user data transmitted from the base station thereafter, and carries out demodulation by a demodulation method corresponding to the modulation method indicated by the transmission parameter received in advance, and carries out decoding based on a decoding method corresponding to the code rate.
The words “downlink” and “uplink” in the downlink data channel, the downlink control channel and the uplink control channel in FIG. 1 are used for channels for signals transmitted to the terminal from the base station and for channels for signals transmitted to the base station from the terminal, respectively. That is, the word “downlink” is used in the names for channels for signals transmitted to the terminal from the base station. Also, the word “uplink” is used in the names for channels for signals transmitted to the base station from the terminal.
Also, the transmission parameter comprises various parameters which are necessary for the transmission of data from the base station to the terminal.
FIG. 2 shows an example of a configuration of a conventional base station that realizes a communications system using adaptive modulation and code rate (adaptive encoding modulation method).
The base station comprises a transmission/reception compatible device 1, an inverse spreading section 2, a power control bit extracting section 3, a control data inserting section 4, a reception quality message extracting section 5, a mode judging section 6, a control section 7, a control data generating section 8, an encoding modulation section 9, a power adjusting section 10, a spreading section 11, an adaptive encoding modulation section 13 and an antenna 14.
The base station demodulates the transmission signal from the user, at the transmission/reception compatible device 1 and the inverse spreading section 2.
That is, a spread spectrum transmission signal is transmitted to the base station from, for example, a terminal as a mobile station capable of radio communications comprising a portable telephone, other PDAs (Personal Digital Assistant) or the like. This transmission signal is received by the antenna 14 and is supplied to the transmission/reception compatible device 1. The transmission/reception compatible device 1 receives the transmission signal from the antenna 14, performs necessary processing and supplies it to the inverse spreading section 2. The inverse spreading section 2 performs an inverse spread spectrum on the signal supplied from the transmission/reception compatible device 1 and supplies it to the power control bit extracting section 3.
The power control bit extracting section 3 extracts a power control bit from the signal supplied from the inverse spreading section 2. In other words, there is included in the transmission signal transmitted to the base station from the terminal a power control bit, which is a one-bit flag that requests an increase or a decrease in the transmission power of the downlink control channel explained in FIG. 1. The power control bit extracting section 3 extracts such a power control bit from the signal supplied from the inverse spreading section 2 and transfers it to the power adjusting section 10.
The power control bit extracting section 3 extracts the power control bit from the signal supplied from the inverse spreading section 2, while at the same time supplying the signal to the reception quality message extracting section 5. The reception quality message extracting section 5 obtains a reception quality message from the signal supplied from the power control bit extracting section 3.
That is, there is included in the transmission signal transmitted to the base station from the terminal a reception quality message that indicates the reception quality (SIR (Signal to Interference Ratio)) at the terminal, as explained in FIG. 1. The reception quality message extracting section 5 obtains, through extraction, the reception quality message from the signal sent from the power control bit extracting section 3, and transfers it to the mode judging section 6.
Here, the signal that is exchanged between the terminal and the base station is composed in frames of a predetermined duration. Moreover, a frame is configured such that, for example, a slot in units of 0.667 msec (milliseconds) is arranged in a plurality of slots. The power control bit mentioned above is such that it is transmitted from the terminal to the base station per slot. Thus, the power control bit extracting section 3 extracts the power control bit for each slot. Also, at the terminal, the reception quality message is such that it is transmitted in units of one frame. Hence, the reception quality message extracting section 5 extracts the reception quality message in units of one frame.
The mode judging section 6 determines the optimal modulation method and code rate from the reception quality message and the condition of the resources the base station has, and assigns encoding resources and power resources to the user (the terminal).
That is, if the combination of modulation method and code rate is now taken to be a transmission mode, the mode judging section 6 determines the transmission mode from the resources of the base station and the reception quality message supplied from the reception quality message extracting section 5, and supplies it to the control section 7.
Here, a large number of transmission modes can be provided through combinations of code rates and modulation methods. However, here, in order to simplify the explanation, three transmission modes #0 to #2 shown in FIG. 3 are explained.
In FIG. 3, R=½ and R=¾ are provided for the code rate (the encoding method). The code rate R=½ signifies that one redundant bit is added for each input data of one bit. The code rate R=¾ signifies that one redundant bit is added for each input data of three bits. In the code rate R=½, although the error correction performance is stronger to the extent that there are more redundant bits in relation to the input data, the number of transmittable data becomes smaller. On the other hand, in the code rate R=¾, although the error correction performance is inferior to the code rate R=½ since the number of redundant bits in relation to the input data is smaller, the number of transmittable data can be increased.
Also, in FIG. 3, QPSK (Quadrature Phase Shift Keying) and 16 QAM (Quadrature Amplitude Modulation) are provided for the modulation methods. As shown in FIGS. 4A and 4B, in QPSK modulation, two bits of encoded data are mapped to one symbol among four symbols (FIG. 4A), and in 16 QAM, four bits of data are mapped to one symbol among sixteen symbols (FIG. 4B). If a transmittable symbol rate is assumed to be constant, data that is actually transmittable is greater for 16 QAM than it is for QPSK. However, in 16 QAM because the distance between the symbols becomes shorter as compared to QPSK, it has a feature that noise characteristics become worse.
In FIG. 3, the combination of R=½ and QPSK, the combination of R=½ and 16 QAM and the combination of R=¾ and 16 QAM are defined as the transmission modes #0, #1 and #2, respectively. Thus, the relationship in terms of data transfer amount would be transmission mode #0 (R=½, QPSK)<transmission mode #1 (R=½, 16 QAM)<transmission mode #2 (R=¾, 16 QAM). On the other hand, the relationship in terms of noise durability characteristics would be transmission mode #0 (R=½, QPSK)>transmission mode #1 (R=½, 16 QAM)>transmission mode #2 (R=¾, 16 QAM).
According to the adaptive encoding modulation method, if the noise is small and the propagation path is good (in a case where the reception quality at the terminal is good), by selecting a combination (a transmission mode) of modulation method and code rate whose data transfer amount is large, it is possible to carry out efficient data transmission. Also, if the noise is large and the propagation path is poor (in a case where the reception quality at the terminal is poor), by selecting a combination of modulation method and code rate whose noise durability characteristics are high, it is possible to suppress data transfer amount and enhance error characteristics.
The mode judging section 6 selects, for example as shown in FIG. 5, a transmission mode in which the error rate of the user data received by the terminal is at or below a predetermined value.
In other words, FIG. 5 shows the relation between the reception quality (SIR) and the error rate of the user data (FER; Frame Error Rate), for each of the three transmission modes #0 (R=½, QPSK), #1 (R=½, 16 QAM) and #3 (R=¾, 16 QAM) mentioned above. The mode judging section 6 judges and selects, for example, a transmission mode in which the error rate of the user data (FER) is 10% or below in relation to reception quality. In this case, the transmission modes #0 (R=½, QPSK), #1 (R=½, 16 QAM) and #2 (R=¾, 16 QAM) are selected respectively at the mode judging section 6 when the reception quality is −8 dB or lower, higher than −8 dB and lower than −4 dB, and −4 dB or higher.
Returning to FIG. 2, the control section 7 transfers the transmission mode determined by the mode judging section 6 to the control data generating section 8 and the adaptive encoding modulation section 13.
The control data generating section 8 generates control data including the transmission mode supplied from the control section 7, and supplies it to the control data inserting section 4.
The control data inserting section 4 is such that besides having the control data from the control data generating section 8 supplied thereto, audio data transferred from a different base station, NW (NetWork) control data used to judge and control a hand-off for shifting control of the terminal from one base station to another base station and the like are sent thereto. The control data inserting section 4 inserts the control data supplied from the control data generating section 8 into the audio data and the NW control data supplied thereto, and supplies it to the encoding modulation section 9.
The encoding modulation section 9 performs an encoding modulation process on the signal supplied from the control data inserting section 4 through a predetermined method, and sends the modulation signal obtained as a result to the power adjusting section 10.
At the power adjusting section 10, the transmission power of the data through the downlink control channel explained in FIG. 1 is determined in accordance with the power control bit supplied from the power control bit extracting section 3. In other words, the power control bit is, for example, as mentioned above, a 1-bit flag, and the power adjusting section 10 processes the modulation signal from the encoding modulation section 9 so as to increase the transmission power in the downlink control channel by 1 dB if the power control bit is 1, and decrease the transmission power in the downlink control channel by 1 dB if the power control bit is 0. Thus, a mechanism for transmitting data in the downlink control channel to the terminal at an optimal power is provided. In addition, the signal in this downlink control channel is transmitted in such a form that it is always associated with the downlink data channel explained in FIG. 1.
Here, in the W-CDMA system, the base station carries out such control of the transmission power in the downlink control channel in accordance with the power control bit transmitted from the terminal with each slot.
The modulation signal whose transmission power is adjusted at the power adjusting section 10 is supplied to the spreading section 11.
On the other hand, packet data in which the user data transmitted through the downlink data channel explained in FIG. 1 is placed is supplied to the adaptive encoding modulation section 13. Then, the adaptive encoding modulation section 13 encodes the packet data in accordance with the code rate represented by the transmission mode supplied from the control section 7, and further carries out a modulation process in accordance with the modulation method represented by the transmission mode. The adaptive encoding modulation section 13 supplies to the spreading section 11 the modulation signal thus obtained by encoding and modulating the packet data.
Here, FIG. 6 shows a configuration example of the adaptive encoding modulation section 13 in a case where, as shown in FIG. 3, the three transmission modes #0 to # 2 are arranged.
The packet data inputted to the adaptive encoding modulation section 13 is supplied to a switch 21.
Then, if the transmission mode supplied from the control section 7 is transmission mode #0, the switch 21 selects a terminal 21a, and a switch 24 selects a terminal 24a. 
The terminal 21a is connected to an encoding section 22a. Therefore, if the transmission mode is #0, the packet data is supplied from the switch 21 to the encoding section 22a. The encoding section 22a encodes the packet data supplied thereto at a code rate of R=½, thereby adding an error correcting code, and supplying the encoded data obtained as a result thereof to a QPSK modulation section 23a. The QPSK modulation section 23a performs QPSK modulation on the encoded data from the encoding section 22a, thereby carrying out a modulation symbol mapping, and supplies a modulation signal obtained as a result thereof to the terminal 24a of the switch 24. If the transmission mode is #0, because the switch 24 has selected the terminal 24a as mentioned above, the modulation signal outputted by the QPSK modulation section 23a is supplied to the spreading section 11 (FIG. 2) via the switch 24.
In addition, if the transmission mode supplied from the control section 7 is transmission mode #1, the switch 21 selects a terminal 21b, and the switch 24 selects a terminal 24b. The terminal 21b is connected to an encoding section 22b. Therefore, if the transmission mode is #1, the packet data is supplied from the switch 21 to the encoding section 22b. The encoding section 22b encodes the packet data supplied thereto at a code rate of R=½, and supplies the encoded data obtained as a result thereof to a 16 QAM modulation section 23b. The 16 QAM modulation section 23b performs 16 QAM modulation on the encoded data from the encoding section 22b, and supplies the modulation signal obtained as a result thereof to the terminal 24b of the switch 24. If the transmission mode is #1, because the switch 24 has selected the terminal 24b as mentioned above, the modulation signal outputted by the 16 QAM modulation section 23b is supplied to the spreading section 11 (FIG. 2) via the switch 24.
Moreover, if the transmission mode supplied from the control section 7 is transmission mode #2, the switch 21 selects a terminal 21c, and the switch 24 selects a terminal 24c. The terminal 21c is connected to an encoding section 22c. Therefore, if the transmission mode is #2, the packet data is supplied from the switch 21 to the encoding section 22c. The encoding section 22c encodes the packet data supplied thereto at a code rate of R=¾, and supplies the encoded data obtained as a result thereof to a 16 QAM modulation section 23c. The 16 QAM modulation section 23c performs 16 QAM modulation on the encoded data from the encoding section 22c, and supplies the modulation signal obtained as a result thereof to the terminal 24c of the switch 24. If the transmission mode is #2, because the switch 24 has selected the terminal 24c as mentioned above, the modulation signal outputted by the 16 QAM modulation section 23c is supplied to the spreading section 11 (FIG. 2) via the switch 24.
Again, returning to FIG. 2, using separate spreading codes, the spreading section 11 performs a spread spectrum on the modulation signal supplied from the power adjusting section 10 and the modulation signal supplied from the adaptive encoding modulation section 13, and supplies the thus obtained spread signals to the transmission/reception compatible device 1. The transmission/reception compatible device 1 performs the necessary process on the spread signals from the spreading section 11, and transmits them to the terminal as radio waves from the antenna 14.
Of the signals thus transmitted, the modulation signal supplied from the power adjusting section 10 is the signal in the downlink control channel of FIG. 1, and the modulation signal supplied from the adaptive encoding modulation section 13 is the signal in the downlink data channel of FIG. 1.
In addition, in the downlink data channel, as mentioned above, the user data is transmitted in the form of packet data. As such, the downlink data channel will hereinafter be referred to as a packet channel as deemed appropriate. Also, the downlink control channel is transmitted in such a manner as to be always associated with the downlink data channel (the packet channel) as mentioned above. As such, the downlink control channel will hereinafter be referred to as an associated channel as deemed appropriate.
Here, the packet channel, in which the user data is transmitted and adaptive encoding modulation is performed, is referred to as, for example, HS-DSCH (High Speed Downlink Shared CHannel). Also, the associated channel, in which the audio data, the NW control data and the control data including the transmission mode are transmitted and on which transmission power control through the power control bit is performed, is referred to as, for example, DPCH (Dedicated Physical CHannel).
Next, FIG. 7 shows a configuration example of a conventional terminal which realizes a communications system using adaptive modulation and coding rate (adaptive encoding modulation method).
A terminal (a user terminal) comprises a transmission/reception compatible device 31, an inverse spreading section 32, an associated channel reception quality estimating section 33, a power control bit generating section 34, a packet channel reception quality estimating section 35, a reception quality message generating section 36, an associated channel demodulation decoding section 37, a control section 38, a user data demodulation decoding section 39, an error detecting section 40, a reception quality message inserting section 43, a power control bit inserting section 44, a spreading section 45, and an antenna 47.
A transmission signal sent out from the base station is received by the antenna 47 and supplied to the inverse spreading section 32 after necessary processing is performed by the transmission/reception compatible device 31. The inverse spreading section 32 performs an inverse spread spectrum on the signal from the transmission/reception compatible device 31 to thereby separate it into a packet channel signal and an associated channel signal. Then, the inverse spreading section 32 supplies the associated channel signal to the associated channel reception quality estimating section 33 and the associated channel demodulation decoding section 37. Moreover, the inverse spreading section 32 supplies the packet channel signal to the packet channel reception quality estimating section 35 and the user data demodulation decoding section 39.
The associated channel reception quality estimating section 33 estimates the signal to noise ratio (SNR) from a pilot signal time-multiplexed on the associated channel. In other words, although not explained in FIG. 2, for example, the control data inserting section 4 is such that it time-multiplexes a predetermined pilot signal as the associated channel signal. Thus, that pilot signal is included in the associated channel signal. The associated channel reception quality estimating section 33 estimates the SNR of the associated channel signal supplied from the inverse spreading section 32 using the pilot signal included in the signal, and supplies to the power control bit generating section 34 the estimated SNR as the reception quality of the associated channel.
The power control bit generating section 34 outputs to the power control bit inserting section 44 a power control bit of a value of 0 if the estimated SNR of the associated channel (the reception quality of the associated channel) is better than a reference quality of the associated channel which is the desired SNR, or a power control bit of a value of 1 if it is worse. Here, the estimation of the SNR at the associated channel reception quality estimating section 33 and the generation of the power control bit at the power control bit generating section 34 are executed for each slot. In the base station in FIG. 2, the power adjusting section 10 controls the transmission power of the associated channel based on the power control bit such that the associated channel can be received by the terminal always at a constant SNR.
The associated channel demodulation decoding section 37 demodulates and decodes the associated channel signal supplied from the inverse spreading section 32 and separates the audio data, the NW control data and the control data. The audio data, the W control data and the control data are supplied to a circuit not shown in the drawings and are also supplied to the control section 38.
The control section 38 detects information on the modulation method and the code rate which are placed in the control data supplied from the associated channel demodulation decoding section 37 and applied to the packet channel, namely, the transmission mode, and carries out mode setting (control) of the user data demodulation decoding section 39.
In other words, if the transmission mode is #0, the control section 38 controls the user data demodulation decoding section 39 so as to QPSK demodulate the packet channel signal and further decode it at a code rate of R=½. Also, if the transmission mode is #1, the control section 38 controls the user data demodulation decoding section 39 so as to 16 QAM demodulate the packet channel signal and further decode it at a code rate of R=½. Alternatively, if the transmission mode is #2, the control section 38 controls the user data demodulation decoding section 39 so as to 16 QAM demodulate the packet channel signal and further decode it at a code rate of R=¾.
On the other hand, the packet channel reception quality estimating section 35 estimates the SNR of the packet channel signal supplied from the inverse spreading section 32. A pilot symbol that is time-multiplexed on the packet channel or a pilot channel symbol transmitted in parallel with the packet channel is used in this SNR estimation.
In other words, although not explained in FIG. 2, the spreading section 11 is such that it time-multiplexes the predetermined pilot signal on the modulation signal supplied from the adaptive encoding modulation section 13 and then carries out a spread spectrum. Thus, the packet channel signal includes the pilot signal. Also, the spreading section 11 is such that it performs a spread spectrum on a different pilot signal with a spreading code different from the spreading code used in the spread spectrum of the modulation signal supplied from the power adjusting section 10 or the adaptive encoding modulation section 13, and transmits it via the transmission/reception compatible device 1 and the antenna 14 in parallel with the packet channel or the associated channel.
The packet channel reception quality estimating section 35 estimates the SNR of the packet channel signal supplied from the inverse spreading section 32 using the pilot signal included in that signal or the pilot signal that is transmitted in parallel with the packet channel signal, and supplies to the reception quality message generating section 36 the estimated SNR as the reception quality of the packet channel.
The reception quality message generating section 36 generates a reception quality message of a predetermined message format representing the estimated SNR of the packet channel (the reception quality of the packet channel) supplied from the packet channel reception quality estimating section 35, and supplies it to the reception quality message inserting section 43.
Here, the estimation of the SNR of the packet channel by the packet channel reception quality estimating section 35 and the generation of the reception quality message by the reception quality message generating section 36 are executed for each frame.
On the other hand, the user data demodulation decoding section 39 decodes and demodulates the packet channel signal supplied from the inverse spreading section 32 in accordance with the control of the control section 38, and supplies the packet data obtained as a result thereof to the error detecting section 40. In addition, the user data demodulation decoding section 39, upon decoding the packet channel signal, carries out error correction of the packet data using the error correcting code included in that signal as a redundant bit.
The error detecting section 40 carries out, for example, parity detection using cyclic redundancy check (CRC) and judges whether or not there is an error in the packet data decoded by the user data demodulation decoding section 39. Then, the error detecting section 40 outputs ACK (ACKnowledge), which is a message indicating that the packet data was received properly, if there is no error in the packet data, and outputs NACK, which is a message indicating that the packet data could not be received properly, if there is an error in the packet data.
In addition, although not shown in FIG. 7 (which similarly applies to FIG. 20 described later), the ACK/NACK outputted by the error detecting section 40 is supplied to the spreading section 45 and is transmitted to the base station.
The reception quality message inserting section 43 frames the reception quality message, which is supplied from the reception quality message generating section 36, in the uplink control channel signal explained in FIG. 1, and supplies it to the power control bit inserting section 44. The power control bit inserting section 44 frames the power control bit, which is supplied from the power control bit generating section 34, in the uplink control channel signal supplied from the reception quality message inserting section 43, and supplies it to the spreading section 45. The spreading section 45 performs a spread spectrum on the uplink control channel signal from the power control bit inserting section 44 and supplies the spread signal obtained as a result thereof to the transmission/reception compatible device 31. The transmission/reception compatible device 31 performs necessary processing on the spread signal from the spreading section 45 and transmits it from the antenna 47 as a radio wave.
In addition, at the terminal, the reception quality message is transmitted per frame, and the power control bit is transmitted per slot.
According to the adaptive encoding modulation method, the data transmission speed can be changed in accordance with the reception condition (the reception quality) of the terminal, and data can be transmitted to the terminal side more efficiently.
By the way, in the adaptive encoding modulation method, for example, the terminal, which is the mobile station, reports the estimated result of the reception quality of the packet channel to the base station, and the base station selects the optimal combination of the modulation method and encoding method based on the reported value (the reception quality indicated by the reception quality message). For this reason, the reception quality accuracy reported to the base station becomes important.
However, because there arises a delay in the estimation and reporting of the reception quality of the packet channel and the message reception at the base station, there are instances where there is some difference between the actual reception quality of the packet channel at the terminal at the time when the base station has demodulated the reception quality message and the reception quality represented by the reception quality message.
In other words, at the terminal, as mentioned above, the reception quality of the packet channel is estimated in periods of frames and is transmitted to the base station. For this reason, from when the reception quality at the terminal is estimated till when that reception quality is recognized at the base station, there is a time lag corresponding to several frames. The reception quality recognized by the base station is a reception quality that is behind by a period of time represented by the time lag. Therefore, there are cases where the reception quality recognized by the base station differs from the current reception quality at the terminal. In such cases, the base station cannot select the optimal combination of the modulation and the encoding methods, which may lower system efficiency.
This phenomenon presents itself most notably when reception propagation path characteristics change rapidly, such as when, in particular, the terminal, which is the mobile station, is moving at high speed.
As such, a method may be considered in which the reception quality message representing the reception quality of the packet channel is transmitted more frequently from the terminal to the base station. However, if the transmission frequency of the reception quality message is made higher, the usage of radio resources increases, and further, power consumption at the terminal also increases.
Therefore, in order to save radio resources and carry out a more effective system operation, it is effective to slow (extend) the reception quality report period. However, by slowing the period, the delay from the time of estimation of the reception quality to when the reported value (the reception quality message) reaches the base station further increases, which results in a larger difference between the reported value and the actual reception quality.